The invention concerns a method and apparatus for measuring metal portion in dross from a dross source.
Dross is formed during production, refinement, alloying and casting of metal, whereby liquid metal reacts with oxygen in air and forms oxides, or from gas treatment, addition of fluxing material, alloying elements or similar. With regard to aluminum, the dross normally comprises from 40 to 90% metallic aluminum, 15 to 45% aluminum oxide and the remainder cryolite, carbides, nitrides, spinel etc.
For example during production and casting of aluminum, a relatively large amount of dross is formed, typically from 0.5 to 10% dross. This dross is withdrawn from the melting furnace at a high temperature, for example 600-800° C., and is in some cases transferred to cooling containers, to dross compressing device for extortion of molten metal, or to an open container for cooling.
In the smelting industry, the dross constitutes a considerable value, and is often sold to other industry parties for extraction of the metal from the dross. During production, dross from numeral furnaces is cooled down and collected in large piles prior to shipping to customer. In order to put as most correct dross price as possible, it is important to know the metal portion in the dross. Since the dross has substantially the same appearance regardless of the metal portion, for example 10% versus 70%, a physical/chemical analysis of the dross must be performed in order to quantify the metal portion.
This can be performed by taking samples of the dross in the dross piles, whereby the metal portion is determined by measuring density with a pycnometer or similar at room temperature and substantially atmospheric pressure. However, this method is highly uncertain because the metal portion varies largely from one furnace to another and even from one operator to another with regard to dross from the same furnace.
Accordingly, there is a need to obtain a more accurate determination of the metal portion in dross, both with regard to quantification of metal portion in dross sold to other enterprises or with regard to the melting process control in order to optimize the process yield.
From the applicant's own WO patent publication 01/20300 it is known that there is a linear relationship between the metal portion in dross and the dross density, represented by the formulam%=k·ρ+a, where m% is percent pure metal, ρ is the dross density, and k and a are empirical constants. However, prior art measurement methods require the sample is cooled down prior to measurement, and it is typically necessary to spend hours to obtain the result of the analysis. However, it is very difficult to control the process parameter with this method in order to control the metal portion in dross and quantify the metal portion with regard to sale of dross to other parties or for use in other processes.
JP patent publication 9178641 describes a method and a container for measuring density of molten metal. A closed container is used, keeping the liquid metal at constant temperature. A sleeve is partly immersed in the liquid metal to form two separate chambers inside and outside the sleeve, respectively. Pressurized inert gas is supplied to the container outside the sleeve, thus establishing a level difference of the metal surface between the interior and exterior of the sleeve. This level difference is, together with the gas pressure inside and outside the sleeve above the metal surfaces, used to calculate the density of the molten metal. However, this technique cannot be used to measure metal in the dross.